The most widespread bonding technology used in the electronics industry is the so-called reflow soldering. The appearance of solder in this technology is solder paste that is a solvent suspension of solder alloy and flux. In mass production solder paste is first applied to the surface of the assembly board (usually woven glass fabric reinforced epoxy resin) or other circuit substrate through a thin metal fixture commonly referred to as a stencil or stencil foil. The stencil foil is a machined metal tool, typically in the form of a sheet, with openings (apertures) on it matching the contact surfaces of the assembly board. During printing a creamy solder paste is smeared onto the assembly board though the openings of the stencil foil so that the solder paste is applied to the assembly board only at the location of these openings. Components are then placed on the assembly board in contact with the solder paste which provides a temporary means of attachment of the components to the assembly board. Soft soldered bonds are then created by application of heat to the solder paste to melt the solder paste which is then allowed to cool to form a soldered bond between the components and the assembly board.
It is important during printing of the solder paste that the stencil foil is uniformly tensioned above the assembly board. Non-uniform tensioning of the stencil foil, as well as any protrusions from the assembly board, can cause a solder paste volume different from the desired volume being applied to the assembly board which can result in shifting of components or short circuits between component leads which may be designed only with small pitch separations. Any level difference on the assembly board, for example, conductive paths covered with solder mask or self-adhesive markers, can also lead to non-contact situations. If these level differences are not too big or necessarily far from the soldering surfaces then the stencil foil may still contact the assembly board and the printed volume may be as designed.
Two typical soldering defects which may occur are ‘solder bridges’ between component leads producing an unwanted contact point and ‘skips’ where electric and/or mechanical contact between the component and the assembly board is not adequately formed. Solder bridges form when an excess amount of solder paste is deposited on the assembly board and skips form when an insufficient amount of solder paste is present.
In the electronic industry stencil foils are used in numbers of hundreds of thousands annually worldwide. Typically all high volume produced electronic circuits have such tools associated with them. As these must be stored through the full lifetime of the circuits they take up huge volumes in electronics manufacturing facilities. To facilitate storage many companies have developed modular solutions that have either stencil foils with openings or narrow sub-frames along the edges to mount the stencil foils into tensioning frames. DE3043666A1 in the name of Messerschmitt shows a solution in which the tension of the frame can be tuned accurately by use of many horizontal and vertical adjusting screws. However, the adjustment operation is burdensome and slow. More practical solutions are provided by frames which can use pneumatic force to release quickly springs arranged around the edges wherein the springs hold the stencil tensioned when there is a lack of pneumatic force applied. Examples of this solution are the frame systems described in GB2317366A in the name of Alpha Fry Limited, WO2013061438A1 in the name of Meiko Electronics Co Ltd and WO2009047012A2 in the name of Dek International GmbH. Despite unquestioned utility of those systems a significant drawback is that they need large numbers of complex shaped, typically metal, components with a requirement for tight manufacturing tolerances leading to a manufacturing cost which is high and the resulting frames may be prone to defects.